1. Field of the Invention
The present invention relates to a crushing method that uses large boreholes in an underwater rock generated by a drop hammer that is suspended from a jib crane of a stone crushing ship, and particularly to a crushing method using large boreholes in an underwater rock in which plural large boreholes are drilled on the rock in a predetermined space, plural free surfaces are formed by the drilled boreholes, and the large drop hammer gives a blow to the free surface of the boreholes, thus the crushing effect is enhanced by the free surfaces of the crushed holes.
2. Description of the Prior Art
When a pier ground work for an underwater bridge structure, a sea structure construction for specific purpose, a development of a waterway for a large ship, and a berth construction for a large ship, the rock existed at the spot in an underway will be eliminated so that the spot has to be flattened, and the main construction can start.
In general, in the elimination work of a spur of a rock, a drilling is executed on the rock, and a blasting powder is loaded in the drill holes. Next the blasting work takes place. However, when a re-blasting work is not possible after the failure of the blasting, or there are structures near the blasting spot, or boulders are produced after the blasting, plural blows are given to the rock to be crushed so as to eliminate the spur of the rock.
In conventional stone crushing work using a blow, a stone crushing ship having a jib crane moves above the rock to be crushed. Anchors of the ship are dropped on the rock that is far away from the hull in a predetermined distance, and the stone crushing ship stops. A drop hammer of 5˜50 tons that hangs from a hoist rope of a jib crane wound on a drum is lifted a predetermined height (15 m above the crush target point). The drop hammer falls free to crush the rock. The above process is repeated.
The crushing method using the crushing drop hammer, compared to the blasting work, has less noise, less shockwave by the vibration and less dispersion of rubble. Therefore, the crushing method is being used where the blasting work is difficult—e.g. where the critical structure such as a gas pipe, a bridge etc. exists near the target rock in the case of a shallow area, and where the blasting fails at an existing blasting point.
However, using the conventional crushing method, since the rock to be crushed is formed as a unitary body, and the blow energy is dispersed to the whole portion of the rock, it is very difficult to break the rock with a repeating blow (e.g. more than 7 times). The crushing speed is very low, and finally it causes non-economic problem that time and investment are excessively required.
Furthermore, the target spot of the rock to be crushed is determined visually, and it is impossible to give a blow to the target spot accurately. This causes to less target effect, and non-economic problems such as an increase of speed and cost for construction process.
On the other hand, when the drilled rock in a underway is crushed by a drop hammer according to a prior art, no drilling operation occurs under 75 mmφ of a borehole, and also the free surface formed around the boreholes has a limit range according to the borehole size. It cannot insure larger free surface than the existed borehole size.
Therefore, to crush the rock, the target blow of the drop hammer requires more than 7 times, which means the not-easy crushing work of the rock in the underwater. Non-efficient problems occur in which construction period and cost for crushing rock should increase.